1. Field of the Invention
The present invention relates to an interferometer, and more specifically to an interferometer configured to measure the shape of an optical surface.
2. Description of the Related Art
An interferometer is used for measuring the shape of the surface of a mirror or a lens with a high degree of accuracy. However, if an aspherical surface is measured with an interferometer that uses a flat surface or a spherical surface as a reference surface, the phase difference between reference light and inspection light is large, and therefore the fringe density of interference fringes is high and exceeds the resolution of a photoelectric conversion element that detects interference fringes. An interferometric scanning method has been proposed in which an inspection surface of an inspection object is moved in the optical axis direction, only a part of the inspection surface is measured at a time, and the measurement results are stitched together to measure the shape of the entire inspection surface. An example of this method is described in Japanese Patent Application Publication No. 2004-534245 (JP 2004-534245), which has also been published in the English language as WO03006920 (A1).
However, in the method described in JP 2004-534245, the total length of an interferometer needs to be long to measure a surface having a large curvature radius. This increases a measuring error due to air fluctuation or vibration. An alternative method is disclosed in Japanese Patent Application Laid-Open No. 2000-97654 (JP 2000-97654) in which a compact point-diffraction interferometer using a reflecting surface is proposed.
The interferometer described in JP 2000-97654 measures the shape of an inspection surface by making an apparent point light source at a position distant from the inspection surface with a reflecting surface. The distance between the apparent point light source and the inspection surface is equal to the curvature radius of the inspection surface.
However, the interferometer described in JP 2000-97654 requires an opening in the inspection surface, which complicates the measurement of the inspection surface.